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Peripheral Nervous System Dr. Gary Mumaugh

Spinal Nerves Overview

Thirty-one pairs of spinal nerves are connected to the spinal cord

No special names; numbered by level of vertebral column at which they emerge from the spinal cavity

o Eight cervical nerve pairs (C1 through C8) o 12 thoracic nerve pairs (T1 through T12) o Five lumbar nerve pairs (L1 through L5) o Five sacral nerve pairs (S1 through S5) o One coccygeal nerve pair

Lumbar, sacral, and coccygeal nerve roots descend from point of origin to the lower end of the spinal cord (level of first lumbar vertebra) before reaching the intervertebral foramina of the respective vertebrae, through which the nerves emerge

Cauda equina describes the appearance of the lower end of the spinal cord and its spinal nerves as a horse’s tail

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Spinal Nerves: Plexuses

Plexus: complex network formed by the ventral rami of most spinal nerves

In plexuses, spinal nerve fibers are rearranged according to their ultimate destination, reducing the number of nerves needed to supply each body part

Four major pairs of plexuses

Cervical plexus C1-C5 o Located deep within the neck to the muscles and skin of the neck, upper

shoulders, and part of the head o Phrenic nerve exits the cervical plexus and innervates the diaphragm

Brachial plexus C5-T1

o Located deep within the shoulder to the lower part of the shoulder and the entire arm

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Lumbar plexus L1-L5 o Located in the lumbar region of the back in the psoas muscle o Formed by intermingling fibers of L1 through L5 o Femoral nerve exits the lumbar plexus, divides into many branches, and

supplies the thigh and leg

Sacral plexus and coccygeal plexus L4-S4

o Located in the pelvic cavity in the anterior surface of the piriformis muscle o Formed by intermingling of fibers from L4 through S4 o Tibial, common peroneal, and sciatic nerves exit the sacral plexus and supply

nearly all the skin of the leg, posterior thigh muscles, and leg and foot muscles

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Dermatomes and Myotomes

Dermatome: region of skin surface area supplied by afferent (sensory) fibers of a given spinal nerve

Myotome: skeletal muscle or muscles supplied by efferent (motor) fibers of a given spinal nerve

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Cranial Nerves Overview

12 pairs of cranial nerves connect to the brain, mostly the brainstem

Identified by name (determined by either distribution or function) or number (order in which they emerge, anterior to posterior) or both

Composed of bundles of axons o Mixed cranial nerve: axons of sensory and motor neurons o Sensory cranial nerve: axons of sensory neurons only o Motor cranial nerve: mainly axons of motor neurons and a small number of

sensory fibers (proprioceptors)

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I Olfactory Nerve

sense of smell

damage causes impaired sense of smell

II Optic Nerve

provides vision

damage causes blindness in part or all of the visual field

III Oculomotor Nerve

controls muscles that turn the eyeball up, down, and medially, as well as controlling the iris, lens, and upper eyelid

damage causes drooping eyelid, dilated pupil, double vision, difficulty focusing and inability to move eye in certain directions

IV Trochlear Nerve

eye movement (superior oblique muscle)

damage causes double vision and inability to rotate eye inferolaterally

V Trigeminal Nerve

largest of the cranial nerves

most important sensory nerve of the face

forks into three divisions: o ophthalmic division (V1) – sensory o maxillary division (V2) – sensory o mandibular division (V3) - mixed

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VI Abducens Nerve

provides eye movement (lateral rectus m.)

damage results in inability to rotate eye laterally and at rest eye rotates medially

VII Facial Nerve

motor – major motor nerve of facial muscles: facial expressions; salivary glands and tear, nasal and palatine glands

sensory - taste on anterior 2/3’s of tongue

damage produces sagging facial muscles and disturbed sense of taste (no sweet and salty)

Five Branches of Facial Nerve o clinical test: test anterior 2/3’s of tongue with substances such as sugar, salt,

vinegar, and quinine; test response of tear glands to ammonia fumes; test motor functions by asking subject to close eyes, smile, whistle, frown, raise eyebrows, etc.

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VIII Vestibulocochlear Nerve

nerve of hearing and equilibrium

damage produces deafness, dizziness, nausea, loss of balance and nystagmus (involuntary rhythms oscillation of the eyes from side to side

IX Glossopharyngeal Nerve

swallowing, salivation, gagging, control of BP and respiration

sensations from posterior 1/3 of tongue

damage results in loss of bitter and sour taste and impaired swallowing X Vagus Nerve

most extensive distribution of any cranial nerve

major role in the control of cardiac, pulmonary, digestive, and urinary function

swallowing, speech, regulation of viscera

damage causes hoarseness or loss of voice, impaired swallowing and fatal if both are cut

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XI Accessory Nerve

swallowing, head, neck and shoulder movement

damage causes impaired head, neck, shoulder movement; head turns towards injured side

XII Hypoglossal Nerve

tongue movements for speech, food manipulation and swallowing

if both are damaged – can’t protrude tongue

if one side is damaged – tongue deviates towards injured side; see ipsilateral atrophy

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Autonomic Nervous System

Autonomic Nervous System Overview

Contains afferent (sensory) and efferent (motor) components (the efferent components are emphasized here)

Major function: to regulate heartbeat, smooth muscle contraction, and glandular secretions to maintain homeostasis

Two efferent divisions: sympathetic division and parasympathetic division

Many autonomic effectors are dually innervated, which allows remarkably precise control of effector

Functions of the autonomic nervous system

Overview of autonomic function o The autonomic nervous system functions to regulate visceral effectors in

ways that tend to maintain or quickly restore homeostasis o Sympathetic and parasympathetic divisions are often exerting antagonistic

influences on visceral effectors o Doubly innervated effectors continually receive both sympathetic and

parasympathetic impulses; summation of the two determines the controlling effect

Functions of the sympathetic division o Under resting conditions, the sympathetic division can act to maintain the

normal functioning of doubly innervated autonomic effectors o Sympathetic impulses function to maintain normal tone of the smooth muscle

in blood vessel walls o Major function of sympathetic division is as an “emergency” system—the

“fight or flight” reaction

Functions of the parasympathetic division o Dominant controller of most autonomic effectors most of the time o Acetylcholine: slows heartbeat and promotes digestion and elimination

Lifespan Changes

Brain cells begin to die before birth

Over average lifetime, brain shrinks 10%

By age 90, frontal cortex has lost half its neurons

Decreased levels of neurotransmitters with age

Fading memory

Slowed responses and reflexes

Increased risk of falling

Changes in sleep patterns that result in fewer sleeping hours

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